We have identified and characterized a new group of LIM-only proteins with four and one half LIM domains (FHL), which are enriched in striated muscle. FHL1 is dramatically upregulated in various models of cardiac hypertrophy and down-regulated in failing heart, suggesting that it plays an important role in these disease settings. Consistent with this, we have generated FHL1 deficient mice and have preliminary data demonstrating that FHL1 deficient mouse hearts display a blunted response to cardiac hypertrophy following pressure overload, without evoking any hemodynamic compromise. FHL1 deficient mice can also completely rescue the increase in cardiac mass in the cardiomyopathic Gq transgenic mouse model. Although the mechanism remains to be determined, yeast two-hybrid screening for FHL1 partners has revealed that one candidate partner is Raf1, a gene previously shown by others to be essential for cardiac hypertrophy and dysfunction induced by pressure overload. FHL1 has also been shown to interact with Erk2, the downstream target of Raf-1. Altogether, these results have led us to the hypothesis that FHL1 is a novel scaffolding protein selectively mediating signaling by the Raf1-Mek1/2-Erk1/2 signaling pathway and essential for pressure-overload mediated cardiac hypertrophy. Accordingly, the Specific Aims are: 1. To define minimal interaction domains within FHL1 and its partners Raf1, MEK1, and ERK2 using yeast- two-hybrid and GST pull-down approaches. 2. To assess effects of FHL1 deficiency or disruption of its interactions with Raf1, MEK1/2, or ERK1/2 on ERK1/2 phosphorylation and/or hypertrophy in both neonatal rat and adult mouse ventricular myocytes. Subcellular localization of each component in this pathway will also be examined. 3. To determine in vivo effects of FHL1 deficiency or disruption of its interactions with RAF1, MEK1/2, or ERK1/2 on long-term pressure overload and Gq overexpression induced hypertrophy. Both FHL1 deficient mice and transgenic mice expressing peptides to block interactions of FHL1 with Raf1, MEK1, or ERK2 will be utilized. The identification and understanding of the FHL1 signaling pathway may ultimately allow us to develop informed therapies to abolish the development of cardiac hypertrophy in human patients with a beneficial outcome for cardiac function.